Composite articles incorporating honeycomb cores are commonly used for fabricating aerospace structures due to their advantageous strength to weight ratio. Honeycomb core composite articles are typically comprised of upper and lower composite skins or layers, i.e., fiber reinforced resin matrix laminates that are separated and stabilized by the honeycomb core. Due to the high bending stiffness and compressive strength properties of honeycomb cores composite articles, i.e., the honeycomb core functions as a shear web and spaces the composite skins from the bending neutral axis, honeycomb core composite articles have particular utility in aerospace applications such as aircraft fuselage panels and door structures. The high strength and low weight of such construction results in lower overall aircraft system weight.
For example, in commercial aircraft, nearly all of the movable control surfaces, wing and tail leading and trailing edge fixed surfaces, doors, and interior cabin structures employ panels formed of honeycomb cores. Although more expensive than simple structures, the honeycomb core panel possess equal strength at higher stiffness, lower weight, and is resistant to higher natural vibration frequencies. Such resistance is very important when structural elements are employed in close proximity to jet and rocket engines. Moreover, the honeycomb core must have small enough cell sizes to provide stabilization of the facings against premature buckling. In addition, the core must be sufficiently tough and abuse resistant to enable the same to be easily handled in a fabrication shop.
The honeycomb core panel possesses equal strength at higher stiffness, lower weight, and is resistant to higher natural vibration frequencies. Such resistance is very important when structural elements are employed in close proximity to jet and rocket engines. Such structural panels generally comprise inner and outer composite skins, which are formed from materials such as Aluminum or composite materials such as fiberglass, graphite, embedded in a resinous matrix, e.g., epoxy, having a honeycomb core material interposed therebetween. Fiber can also be constructed of any other materials having a very small diameter and high strength and stiffness. Resins may typically consist of an epoxy, polycyanate, bismaleimide, and the like. The strength and stiffness of the resin matrix also affects the strength of the finished composite structure. For example, stronger resins such as epoxies usually yield a higher strength composite structure than lower strength resins such as polyester.
Where high damage tolerance and abuse resistance are requires, Aramid honeycombs are employed. For example, KEVLAR® fiber can be used advantageously for this purpose. KEVLAR® is a registered trademark of E. I. du Pont de Nemours & Co., Wilmington, Del. for an aromatic polyamide fiber of high tensile strength. Besides weight and strength, KEVLAR® has a slightly negative axial coefficient of thermal expansion similar to graphite, which means KEVLAR® laminates can be made thermally stable. Unlike graphite, KEVLAR® is very resistant to impact and abrasion damage and can be used as a protective layer on graphite laminates.